When running reactions in
the organic chemistry lab, you want to have a
way to monitor them. One of those methods is known
as thin-layer chromatography, or TLC for short. This works like all other
kinds of chromatography in that you have a mobile
phase and a stationary phase, and the compounds that you're
trying to separate interact with these two phases. First, let's talk about
the stationary phase. In TLC, you usually
have a plate that's completely coated
with silica gel. The silica gel is
the stationary phase. There are other
materials you could use, but silica gel is
the most common. What you would do
first is take a spotter and dip into your
reaction flask, then put a little spot here. You want to find out whether
is there one compound in there or are there multiple compounds. Next, what you'll need is
to prepare the mobile phase. This is done in something known
as the developing chamber shown here in white. First, what you would
do is put in a tiny bit of your mobile
phase, meaning it can be any solvent or any
mixture of solvents, and you want to keep this to
be a relatively small amount. If the level of
this is too high, your spot will be completely
submerged in this. You'll want to put in
a piece of paper that will indicate whether or not
this chamber is saturated. When setting this
up, you want to check that this piece of paper is wet. This lets you know that the
vapors from the mobile phase are completely everywhere
in the developing chamber. Next, what you do is,
you take your TLC plate and put it inside here. Finally, you would
close the top. You want to do this because
if you didn't and you left the top open, organic solvents
are very volatile, meaning that they readily evaporate
from liquid into gas. Next, what would you observe? Well, let's think about
what could happen. You have your pink mobile
phase, and what will happen next is that it'll travel up the TLC
plate through capillary action. Capillary action is
just a term for when you have some kind of
solid thing like silica gel that sucks up a liquid. And when you see that
it's gotten pretty close to the edge, say
around here, you'll want to pull this out of
the developing chamber. Make sure to use
a pencil to mark where your mobile
phase got up to. How come we can't see anything? That's because usually you
need something like a UV lamp shining on this, so that
you can visualize something. So let's say we take our UV
lamp and shine it on here. Compounds that are aromatic will
usually show up and fluoresce. And let's say that we
had these two dots. So what does that tell us? That tells us that whatever
was in our reaction flask was a two-component
mixture and that there's at least two compounds in there. However, you would
have seen more dots if there were three compounds
or four compounds or even more. TLC plates can get really messy
when you're doing research. But what can we tell
about these two compounds? So TLC is a pretty
qualitative method. It'll tell us whether things
are more polar or less polar, so what we have
to keep in mind is that the stationary phase, where
the silica gel-- silica gel is very, very polar. So you can see this one
didn't move to far-- it means it must have been
really attracted to the silica gel-- but this one
moved a lot more, so this is less polar and more
attracted to the mobile phase. So we still don't
really know what compounds these are exactly. But let's say that you
knew that inside the flask you had naphthalene
and benzoic acid. How can we tell which
spots these correspond to? Well, you'd want to look
at them and figure out which one is more polar and
which one is less polar. As you can see,
naphthalene is just made out of carbons
and hydrogens, so it's pretty nonpolar. So we can indicate that
this corresponds with this. And because of the carboxyl
group in the benzoic acid, this group right
here, you can tell it's a pretty polar molecule. So this would
correspond to this. And that's how you
do a TLC in a lab. Again, let's review. You first have your plate
that you put a little spot on; you put it into the
developing chamber, which contains the mobile
phase; wait a little bit for the mobile phase to
travel upwards; pull it out of the chamber; and then use
a UV lamp to see what spots are there; and then try
to compare those two spots in terms of polarity.